Autism Spectrum Disorder (ASD) is a behaviorally diagnosed disorder of early onset characterized by impairments in social communication and restricted and repetitive behaviors. While accurate diagnosis can reliably be made by 24 months and signs of the disorder are present even earlier, the majority of children with ASD are not diagnosed until after age 4. Some of the earliest signs of ASD involve auditory processing deficits, including absent or atypical response to speech (such as one's name being called). Testing early auditory processing is difficult in very young, minimally verbal children, but the integrity of outer hair cell function can be evaluated reliably using otoacoustic emissions (OAEs). Sound causes contractions of the outer hair cells and generates acoustic signals (OAEs), which can be recorded in the external ear canal. Measuring these signals is noninvasive and reliable, and is a routine approach to testing auditory functioning in children as young as infancy. We have recently shown that high functioning children and adolescents with ASD (with normal hearing thresholds) have reduced OAEs using multiple tests in the 1 kHz frequency region critical for speech, yet the other frequencies tested between 0.5-8 kHz were unaffected. Moreover, we found that lower OAEs in this 1 kHz region were significantly related to greater autism severity in the ASD group. OAEs can also be used to measure the strength of descending pathways from the midbrain to the cochlea. This olivocochlear [OC] efferent system has cell bodies found in the superior olivary complex, which has previously been found to be either absent, greatly reduced, or disorganized in autopsy and MRI studies of adults with ASD. This OC efferent feedback system can be activated with sound and because the medial OC (MOC) system projects to outer hair cells, it is possible to measure efferent feedback processes at the cochlear periphery using OAEs. It has previously been shown that children with ASD have reduced MOC efferent feedback strength and greater right/left (R/L) asymmetries in their responses. In Aim 1, we will investigate baseline cochlear function using two different OAE measures, in young children (4-6 yrs) with ASD, including minimally verbal and verbal subsamples, and age-matched typical controls. We predict that children with ASD will have reduced OAEs in the 1 kHz speech frequency region, with minimally verbal children having more severely reduced OAEs than those with better early language skills. In Aim 2, we will measure MOC efferent feedback strength, again using two different OAE-based tests with binaural stimulation, and test the hypothesis that MOC efferent feedback strength and R/L ear symmetry are impaired in ASD in the 1 kHz speech frequency region, and that these will be more pronounced in minimally verbal children. Information gained from these studies will allow us to determine if non-invasive measures of cochlear function and MOC efferent feedback may serve as objective early indicators of auditory processing differences in ASD.!